Sensory

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Chapter 10
Sensory Physiology
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Sensory Receptors

• Change environmental info into APs -- the common
  language of NS
• Each type responds to a particular stimulus (e.g.
  sound, light, temperature, pressure)
• Different stimuli are perceived as different
  because of the CNS pathways they activate

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Sensory Receptors continued

• Category types
• Chemoreceptors sense chemical stimuli
• Photoreceptors transduce light
• Thermoreceptors respond to temperature changes
• Mechanoreceptors respond to deformation of their
  cell membrane
• Nociceptors respond to intense stimuli (damaged
  cells) by signaling pain
• Proprioceptors signal positional info of body
  parts

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Sensory Receptor Responses

• Tonic receptors respond at constant rate as long
  as stimulus is applied
• e.g. pain
• Phasic receptors respond with burst of activity
  but quickly reduce firing rate to constant
  stimulation (adaptation)
• e.g. smell, touch

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Cutaneous Receptors

• Nerve endings may be naked fibers (hot, cold,
  pain, touch, pressure),
• or encapsulated (touch, pressure)

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Receptive Field
The Two-Point Touch Threshold

• Is minimum distance at which 2 points of touch
  can be perceived as separate
• Measure of tactile acuity or distance between
  receptive fields

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Lateral Inhibition

• CNS process that sharpens sensation
• Sensory neurons at center of stimulation area
  inhibit more lateral neurons
• e.g. when blunt object touches skin sensory
  neurons in center are stimulated more than outer
  ones

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Taste/Gustation

• Detects sweet, sour, salty, bitter and amino
  acids (umami)
• Taste receptor cells are modified epithelial
  cells
• 50-100 are in each taste bud

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Taste/Gustation

• Salty and sour do not have membrane receptors
  act by passing thru membrane channels
• Sweet and bitter have membrane receptors act
  thru G-proteins

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Smell (Olfaction)

• Olfactory Receptors are located in olfactory
  epithelium at top of nose (Cranial nerve I,
  Olfactory nerve)

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Auditory/ Equilibrium SensationsVestibular
Apparatus

• Provides sense of equilibrium
• orientation to gravity
• Vestibular apparatus and cochlea form inner ear
• V. apparatus consists of otolith organs (utricle
  and saccule) and semicircular canals

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Vestibular Apparatus continued

• Utricle and saccule provide info about linear
  acceleration
• Semicircular canals, oriented in 3 planes, give
  sense of angular acceleration

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Vestibular Apparatus continued

• Hair cells are receptors for equilibrium
• Each contains 20-50 hairlike extensions called
  stereocilia
• 1 of these is a kinocilium

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Hearing The Outer Ear

• Sound waves funneled by pinna (auricle) into
  external auditory meatus
• External auditory meatus funnels sound waves to
  tympanic membrane

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Ears and Hearing - Middle Ear

• Middle ear is between tympanic membrane and
  cochlea holds ossicles

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Hearing The Middle Ear

• Malleus (hammer) is attached to tympanic membrane
• Carries vibrations to incus (anvil)
• Stapes (stirrup) receives vibrations from incus,
  transmits to oval window

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Ears and Hearing - Middle Ear continued

• Stapedius muscle, attached to stapes, provides
  protection from loud noises
• Can contract and dampen large vibrations
• Prevents nerve damage in cochlea

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Ears and Hearing - Cochlea
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Ears and Hearing - Cochlea continued
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Hearing The Cochlea continued

• High frequencies produce maximum stimulation of
  Organ of Corti closer to base of cochlea and
  lower frequencies stimulate closer to apex

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Neural Pathway for Hearing

• Info from 8th cranial (Auditory) nerve goes to
  medulla, then to inferior colliculus, then to
  thalamus, and on to auditory cortex of temporal
  lobe

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Neural Pathways for Hearing
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Hearing Impairments

• Conduction deafness occurs when transmission of
  sound waves to oval window is impaired
• Helped by hearing aids
• Sensorineural (perceptive) deafness is impaired
  transmission of nerve impulses
• Helped by cochlear implants

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Vision
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Visual Field- Refraction

• Refraction is the bending of light by cornea and
  lens
• Image projected onto retina is upside down and
  backward

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Accommodation

• Ability of eyes to keep image focused on retina
  as distance between eyes and object changes
• Results from contraction of ciliary muscle

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Visual Acuity

• Sharpness of vision
• With myopia (nearsightedness) image is focused in
  front of retina because eyeball is too long
• With hyperopia (farsightedness) image is focused
  behind retina because eyeball too short

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Visual Acuity continued

• With astigmatism cornea or lens is not
  symmetrical
• Light is bent unevenly

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Retinal Organization

• Is a multilayered epithelium consisting of
  neurons, pigmented epithelium, and
  photoreceptors (rods and cones)

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Retina continued

• Rods and cones face away from pupil
• send sensory info to bipolar cells
• Bipolars to ganglion cells
• Ganglion cells project axons thru optic nerve to
  brain
• Horizontal cells and amacrine cells are
  interneurons

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Effect of Light on Rods

• Rods are activated when light produces chemical
  change in rhodopsin
• Causes it to dissociate into retinal and opsin
• Causes changes in permeability, resulting in APs
  in ganglion cells

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Cones and Color Vision

• Cones less sensitive to light than rods
• Provide color vision and greater visual acuity
• In day, high light intensity bleaches out rods,
  and high acuity color vision is provided by cones

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Cones and Color Vision continued

• Humans have trichromatic color vision
• All colors created by stimulation of 3 types of
  cones
• Blue, green, red
• According to region of visual spectrum they absorb

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Cones and Color Vision continued

• Instead of opsin, cones have photopsins
• A different photopsin for each type of cone
• Causing each to absorb at different wavelengths

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Visual Acuity and Sensitivity

• Eyes oriented so that object of attention is
  focused on fovea centralis
• Pin-sized pit within yellow macula lutea
• Contain only cones
• Neural layers displaced to sides so light strikes
  cones directly

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Visual Acuity and Sensitivity continued

• In fovea each cone supplies 1 ganglion cell (1 to
  1 ratio)
• Allows high acuity
• Peripheral regions contain both rods and cones
• Degree of convergence of rods on ganglions is
  much greater
• Allows high sensitivity, low acuity

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Visual Field

• Cornea and lens focus right part of visual field
  on left half of retina
• Left half of visual field focuses on right half
  of each retina

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Neural Pathways from Retina

• Left lateral geniculate nucleus receives input
  from right half of visual field of both eyes
• Right Lateral geniculate nucleus receives input
  from left half of visual field of both eyes

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Eye Movements

• Superior colliculi coordinate
• Smooth pursuit movements track moving objects,
  keeping image focused on fovea
• Saccadic eye movements allow eyes to jump from
  one object to another
• Such as when reading words
• Even during a fixed gaze there are tiny
  fixational movements that prevent photoreceptor
  bleaching
• Pupillary reflex constricts pupil in response to
  strong light

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